This invention relates to a multiwire conductor and, in particular, to a multiwire conductor comprising at least one center wire laid substantially parallel to the axis of the conductor with one or more layers of wires stranded around it in a normal manner.
One form of such a conductor is disclosed in my prior U.S. Pat. No. 3,180,925.
The object of this invention is to provide a multiwire conductor particularly suited for use in electric cables, which are expected to maintain circuit integrity under high temperature conditions, and in electric strain cables.
A cable having an insulation which, when it is subjected to very high temperatures or even flames, is transformed into an electrically insulating ash, may maintain its working capacity during and after a fire. An assumption is then that the insulating ash is kept safe in position within the cable, and also that the cable is fastened securely to constructions being stable during the fire to avoid bending and mechanical stress of the cable.
Even when all such assumptions are fulfilled, known cables have evidenced a high percentage of failures during flame tests. To understand the background of such failures, it is necessary to consider the design of a conventional cable within this art.
A conventional stranded multiwire conductor comprises at least seven copper wires of equal diameters with six of the wires stranded helically and evenly around the seventh wire, which is centrally arranged in the multiwire conductor. The center wire is straight and linear.
The known multiwire conductor is usually insulated with an inorganic insulation or an insulation comprising a large inorganic component. To maintain the insulation securely around the conductor, each insulated multiwire conductor is covered by a glass braid. Over the glass braid a metallic shield may be applied, such as one consisting of a copper braid. It is further known to use silicone rubber as conductor insulation and to cure the silicone rubber at elevated temperatures before applying the glass braid.
From the above-mentioned U.S. Pat. No. 3,180,925, it is further known to envelop the insulated multiwire conductor in a glass braid and to use molybdenum in the center wire in the multiwire conductor. A cable may comprise several individually insulated multiwire conductors in a common metallic shield.
When using a design according to said U.S. Patent, it has been found that failures do not occur. However, it is not an ideal solution to use one conductor of molybdenum in such a multiwire conductor. On the one hand, molybdenum is a rather expensive material, and on the other hand, there is a desire that all the wires should be made of the same material, as this will simplify jointing in the field.
One theory of how the failures occur when a conventional cable, not including a molybdenum center wire, is subjected to a flame test is stated below.
When the insulation, as e.g. silicone rubber, is burnt into ashes, the insulation diameter tends to increase slightly. The braid of glass, which is applied around the core, then is radially extended, and this again results in a longitudinal contraction of the braid. As this longitudinal contraction occurs simultaneously with the radial expansion, the frictional forces between the burnt insulation and the multiwire conductor elements become very large. Therefore, the longitudinal contraction of the braid will be frictionally transferred to the helical, outer elements of the multiwire conductor and further to the center wire. Because of the helical form, the outer elements of the multiwire conductor are free to contract in the longitudinal direction. The center wire, on the other hand, in a conventional multiwire conductor, is a straight element, and an even contraction in longitudinal direction beyond the elasticity limit is impossible. Instead, there will be built up a longitudinal, compression force until the center wire kinks in an uncontrolled manner. The other elements of the multiwire conductor will then also be forced out in a kink and may, if a large kink occurs, make contact with the outer metal braid, which finally results in a short.
The object of the present invention is to eliminate the risk of kinks forming in the conductors of cables which have the ability to stay serviceable (without shorting) even after burning. The same design may also be used to prevent kinks in the conductors of so-called strain cables (cables which are designed to withstand high tension). The conductors of such cables have been known to form kinks when the high tension is removed, much in the same manner as described above, only that the compressive force causing the kinks to form is a result of the cable "snapping back" due to its elasticity.
It is well known that the risk of forming kinks in the conductors of a strain cable is reduced by using, for instance, a plastic monofilament instead of the center wire in a seven wire conductor. A similar result will be achieved according to the present invention without resorting to a different material in the center of the conductor by arranging or preparing the center wire in a manner to be described later herein. When jointing is considered, it is advantageous to have the same material in all the wires of the multiwire conductor.
When a multiwire-conductor having a molybdenum center wire works properly, this may be a result of the fact that the molybdenum has a temperature coefficient which differs from that of copper.
The object of the present invention, therefore, is to achieve a multiwire conductor which neither exhibits large kinks when subjected to flame tests, nor when used in a strain cable, and where all the elements of the multiwire conductor consist of the same metallic material to simplify the jointing procedures.
It is not certain that the simple theory stated above represents a complete view of this problem, but experiments have at least proved that a cable according to the present invention endures flame tests without any failures.